1. Field of the Invention
This invention relates to apparatus for the purification of a liquid, such as water, by reverse osmosis.
2. Prior Art
In the specification of U.S. Pat. No. 4,124,488 there is described an apparatus for the reverse osmosis purification of water or other fluid comprising a module including a reverse osmotic membrane, a fluid inlet and fluid outlet for passage of fluid through the module over one surface of the membrane and an outlet for the passage of purified fluid out of the module from the opposite surface of the membrane, together with a ram having a piston or diaphragm in a cylinder for forcing fluid from the front face of the piston or diaphragm through a valve to the fluid inlet of the module, the ram having an operating rod extending outwardly from the rear face of the piston or diaphragm and means, including a first controlled valve connecting said fluid outlet from the module to the cylinder to admit returned fluid from the module onto the rear face of the piston or diaphragm and a second control valve to control discharge from the rear of the piston or diaphragm. Such apparatus will be referred to hereinafter as apparatus of the kind described.
With this construction, a fluid such as seawater is forced by the ram into the module. A valve is provided between the ram and the module to prevent return of fluid from the module on the return stroke of the ram; this may be a non-return valve or it may be a valve which is controlled by the fluid pressure or by the movement of the ram or in synchronism therewith. On the forward stroke of the ram, some water may pass through the membrane and the remainder of the fluid is returned to the rear face of the piston or diaphragm.
As is explained in the aforementioned specification, with this arrangement, fluid may be forced into the module at a very high pressure such that water purification can be obtained. For brackish water, a pressure of the order of 300 to 500 p.s.i. would be necessary whilst for seawater, since the osmotic pressure is higher, a pressure typically of the order of 600 to 1000 p.s.i. might be necessary. The construction described above enables these high pressures to be obtained economically and efficiently because the return fluid from the membrane is applied to the rear surface of the piston or diaphragm. This rear surface, because of the presence of the operating rod, has a slightly smaller effective area. The pressure in the system builds up until a pressure is reached at which water will pass through the membrane on each stroke in equivalent volume to the difference in volume between the rear and front ends of the cylinder due to the presence of the operating rod. Power has to be supplied to the operating rod and the required work for each stroke depends only on the difference of the front and rear face areas, the pressure and the length of the stroke. The device is thus self-regulating and tends to operate in a condition where the proportion of water passing through the membrane to the total inlet fluid is equal to the ratio of the operating rod cross-section to the piston or diaphragm front face area. There is thus no need for any pressure regulation by relief valves. No adjustment is required for variations of salinity and the same equipment may be used for seawater as for slightly brackish water. Although it is convenient to refer to water, the apparatus may be used for reverse osmosis treatment of other fluids.
Reference may also be made to the following further U.S. Pat. Nos. 1,909,145; 3,405,058; 3,493,495; 3,498,910; 3,825,122.